Functionality; Operation Principle - ABB RELION 611 Series Technical Manual

1MRS757454 D
4.4.4.3
4.4.4.4
611 series
Technical Manual

Functionality

The negative-sequence overcurrent protection for machines function MNSPTOC
protects electric motors from phase unbalance. A small voltage unbalance can
produce a large negative-sequence current flow in the motor. For example, a 5 percent
voltage unbalance produces a stator negative-sequence current of 30 percent of the
full load current, which can severely heat the motor. MNSPTOC detects the large
negative-sequence current and disconnects the motor.
The function contains a blocking functionality. It is possible to block the function
outputs, timers or the function itself, if desired.

Operation principle

The function can be enabled and disabled with the Operation setting. The
corresponding parameter values are "On" and "Off".
The operation of MNSPTOC can be described by using a module diagram. All the
modules in the diagram are explained in the next sections.
GUID-F890E844-B9C9-4E99-A51F-6EAB19B5239B V1 EN
Figure 117: Functional module diagram
Level detector
The calculated negative-sequence current is compared to the Start value setting. If the
measured value exceeds the Start value setting, the function activates the timer
module.
Timer
Once activated, the timer activates the START output. Depending on the value of the
set Operating curve type, the time characteristics are according to DT or IDMT. When
the operation timer has reached the value set by Operate delay time in the DT mode
or the maximum value defined by the inverse time curve, the OPERATE output is
activated.
In a drop-off situation, that is, when the value of the negative-sequence current drops
below the Start value setting, the reset timer is activated and the START output resets
after the time delay of Reset delay time for the DT characteristics. For IDMT, the reset
time depends on the curve type selected.
Section 4
Protection functions
243